Method for making silver/cadmium oxide contact materials



United States Patent O 3,506,437 METHOD FOR MAKING SILVER/CADMIUM OXIDECONTACT MATERIALS Childress B. Gwyn, Jr., Wethersfield, Conn., assignor,by mesne assignments, to Textron Inc., Providence, R.I., a corporationof Delaware No Drawing. Filed Nov. 7, 1967, Ser. No. 681,097 Int. Cl.C22c 5/00, 1 H01b 1/02 U.S. Cl. 75-206 5 Claims ABSTRACT OF DISCLOSURE Amethod for the manufacture of silver/ cadmium oxide electrical contactmaterials, in which a silver/cadmium alloy material is reduced todiscrete flat particles having thicknesses not more than about 5microns. The discrete particles are then oxidized to form asubstantially homogeneous silver/ cadmium oxide material.

BACKGROUND OF THE INVENTION (1) Field of the invention carbonate to theoxide. An added disadvantage of the latter two methods is the fact thatparticle shrinkage during the course of the oxidation-reductionreactions is largely unpredictable.

Yet a fourth method, that of oxidizing silver/ cadmium alloys of thedesired composition, in the form of the desired finished contactelements or as shot which may, after compacting, be extruded into theshapes of such elements (see, for example, Haarbye U.S. Patent No.3,317,991, granted May 9, 1967), is costly in its energy requirementsdue to the prolonged periods of heating required and the, nevertheless,incomplete oxidation of the cadmium metal component thus effected.

SUMMARY OF THE INVENTION The present invention is directed to aneconomical method for making substantially homogeneous silver/ cadmiumoxide electrical contact materials, which method involves the physicalreduction of a silver/ cadmium alloy feed material of the desiredcomposition to discrete, thin particles whose major dimensions, e.g.,length, are at least 3 times their thicknesses, and which havesubstantially uniform thicknesses of no more than about 5 microns. Theformation of such fiat, relatively thin alloy particles which may, forexample, be provided in either platelet or thread-like form, facilitatessubsequent uni- While the electrical properties of such mixtures, whichA number of processes have heretofore been proposed to avoid materialinhomogeneity of silver/ cadmium oxide electrical contact materials, andto improve the uneconomical methods of achieving such homogeneity.

One such process involves the sintering of a powder mixture of silverand cadmium oxide, as described by Goetzel in his Treatisebn PowderMetallurgy, vol. III, p. 647 (1952). Such technique, while usingrelatively inexpensive materials, does not result in a compositematerial of the required homogeneity due to the differences intheshapes, the sizes, and the specific gravities, of the particlesconstituting the initial powder mixture.

Another method prescribes the use of an initial mixture of silver andcadmium powders,the cadmium being subsequently oxidized in a suitablearnosphere. In such process difficulties arise from the fact that oxygendiffusion into the cadmium particles, needed for effective oxidation toa certain minimal depth, is slow, so that in actual practice anunoxidized core remains in the cadmium powder. particles. Purecadmium-metal does not possess those characteristics requisite foranonform oxidation of the cadmium component of the alloy 'whereby toproduce a substantially homogeneous silver/ cadmium oxide contactmaterial. Materials thus produced by the successive physical forming andoxidation steps may thereafter be compacted into the final desired shapefor the electrical contact element in question and sintered, if desired.

The method of this invention thus provides an economical technique forproducing silver/cadmium oxide electrical contact materials ofsubstantially homogeneous composition, which materials may thus beemployed to produce non-welding electrical contact elements. It will beunderstood that such homogeneity is achieved by the initial formation ofdiscrete, relatively thin silver/cadmium alloy particles, the decreasedthicknessof such particles reducing the depth of necessary oxygendiffusion therethrough during oxidation and thereby facilitating thesubsequent formation of substantially homogeneous silver/cadmium oxidematerials. It will be understood that various procedures for forming thediscrete silver/ cadmium particles having thicknesses of no more than 5microns, generally from about 1 to 3 microns, may be utilized within thescope of the present invention. While several preferred techniques aredescribed hereinafter for forming alloy particles of the desiredconfiguration, it is intended that the method hereof encompasses anysuch techniques for converting the initial silver/cadmium alloy materialto the desired configuration set forth hereinabove, facilitating theformation of substantially homogeneous oxidized electrical contactmaterials.

PREFERRED EMBODIMENTS OF THE INVENTION Homogeneous silver/cadmium oxidecontact materials are produced in accordance herewith from silver/cadmium alloy materials having compositions known in the art. While theexact proportions of the silver and cadmium constituents of such alloymaterials may be dictated by the electrical and/or mechanicalrequirements for the desired final contact material, it is preferred toutilize alloys incorporating quantities of the order of by weight silverand 10% by weight cadmium, good results having been obtained by the useof alloys constituted of from about 80 to by weight silver and,correspondingl 20 to 5%. by weight cadmium.

As indicated hereinabove, the alloy materials (which.

may initially be provided in ingot or any other desired form) areconverted, in accordance with this invention, to discrete, thinparticles which may subsequently be oxidized to form substantiallyhomogeneous silver/cadmium oxide compositions. The discrete particlesthus formed have large surface area to volume ratios, in excess of 3 to1, and have major dimensions (lengths or diameters) of at least .005,and generally from about 3 to 20 times their thicknesses. Suchparticles, which may possess configurations of extremely thin flakes orplatelets, or finely-spun threads, have substantially uniform thicknesses of no more than about 5, and preferably from about 1 to 3,microns. The formation and use of such particles has been found uniquelysuited to the preparation of substantially homogeneous contactmaterials.

Forming of the silver/cadmium alloy stock material may be achieved byvarious techniques as, for example, by impinging a molten stream of thealloy against a suitable baflle to disintegrate the alloy into discreteparticles and flatten and/or comminute such particles into the desiredconfiguration described hereinabove. Alternatively, the solid alloymaterial, in bar, rod or other desired form, may be mechanically abradedto remove flake-like particles of the desired material suitable foroxidation in accordance herewith. Thirdly, the malleable alloy materialmay be reduced to the desired discrete, thin paroxygen moleculcshavingonly. relatively short distances to diffuse into the center of theindividual granules treated as compared with spherical'granules of equalvolume.

While it is believed that formation of substantially homogeneoussilver/cadmium oxide contact materials by the-present process isachieved by the mechanismproposed hereinabove, it will be understoodthat the invention is not limitedto such proposed mechanism. Rather,.itis intended that the present process shall not be limited other than asdefined in the claims appended hereto;

The following'examplesfurther'illustrate preferred embodiments of thepresent process for forming silver/cadmiumoxide electrical contactmaterials: i

EXAMPLE 1 A silver/cadmium alloy'is melted by a source of heatsufficient for'the purpose, and the molten metal alloy is then pressedthrough a nozzleinto a chamber for expansion into a jet of finedroplets. This jet impinges upon a water barrier, acting as a baifle forinstantly cooling and disintegrating the molten stream impingingthereon. As a result, the alloy solidifies into flat platelets-ofsilver/ cadmium alloy having thicknesses less than 5 microns.

' The particles are then collected, dried, and oxidized in a ticulateform by repeated rolling with no intermediate annealing operations.Alternatively, the physical con version operation may be effected byspinning the molten silver/cadmium alloy material into strands orthreads of the desired shape.

The discrete silver/cadmium particles having large surface to volumeratios are thereafter readily oxidized to form the desiredsilver/cadmium oxide material. Such oxidation, which requiresconsiderably less time than the partial oxidation techniques heretoforeemployed, may be effected by heating the alloy composition attemperatures of from about 1300 to 1500 F. for periods of from about 2to 24 hours. The discrete particles are thus oxidized substantiallyuniformly throughout, no appreciable core of unoxidized cadmium. metalremaining.

The silver/ cadmium oxide contact material thus formed may be readilycompacted, extruded into wire or other shapes, and interlocked intodesirable forms suitable for electrical contact applications. In variousinstances the contact materials may be used directly as compactedwhereas in other instances it may be desirable to subsequently sinterand/or re-coin or compact the material for particular intended uses. Inany event, the contact materials thus produced are characterized byhomogeneity, high density, mechanical strength, and superior electricalcharacteristics. Moreover, as noted hereinabove, such materials may beformed at lower cost and in decreased time as compared with priortechniques employed for converting silver/cadmium alloy stock materialsto silver/cadmium oxide electrical contact components.

It is believed that the improved homogeneity effected in accordance withthe present invention is produced by virtue of the use of the discrete,thin particles formed in accordance herewith. conventionally made alloyshot, powder or granules are composed of particles which are largelyspherical. It is a well known fact that a sphere has the smallestsurface area of any body relative to its volume. Since, however,oxidation proceeds by diffusion from the surface inwardly of the body tobe oxidized, it is desirable, in the present instance, to rather oxidizealloy materials-having large surface to volume ratios or, otherwisestated, large surface to cross-section ratios. Such is achieved inaccordance with the present process by the formation of thin flakes,platelets or strands of the silver/cadmium alloy which have thicknessesof no more than 5 microns and length or diameter tothickness f; I

chamber at 1470 F., containing an oxygen-rich atmosphere.

In one experiment flakes incorporating 90.5 weightpercent silver and 9.5weight percent cadmium, having a thickness 'of between 2 and 5 microns,were oxidized to substantially 100 of the flakes cadmium content at atemperature of 1550" F. within a period of 10 hours. By varyingthethickness of the particles by means of the parameters of the sprayapparatus, e.g., the pressure, temperature, nozzle diameter, jetvelocity, composition of the coolant, the time and temperature requiredfor complete oxidation may be varied at will.

The above processing time compares favorably with that required foroxidizing a silver/ cadmium bar of similar composition and of dimensions/1 inch thick x 2 inches wide x 10 inches long, which may require from75 to 500 hours at 1500 F; to produce an internal oxidation of between50% and ofthe cadmium.

EXAMPLE 2 The silver/cadmium alloy of Example 1 isconverted to agranulate form, and the granules are propelled at great velocity"against a hard surface, such as a stainless steel plate. By repetitionof such procedure, the granular particles are reduced in size as Well asflattened, the-number of repetitions'of the, process being dictated bythe actual particle size found suitable for the particular conditionsprevailingin the oxidation chamber used. Oxidation is effected in thesame manner as described in Example 1. Representative dimensions'offinal particles are mesh, or in the range of l to 3 microns thickness. I

EXAMPLE 3 A solid body of silver/cadmium alloy of the above compositionis subjected to'abrasionby a file, a rasp, or similar tool capableofreducing the alloy to chips or filings or powder withoutitselfcontributing .to the granulate thus formed. Inone" proposed embodimentthe tool and the alloy body may usefully be wheel shaped and rotated inthe same direction. while their circumferential faces are in contactunder pressure, in order to facilitate the'continuous production ofthegranulate. This granulate may either be feddirectly to the oxidizingchamber, or it may be subjected to further treatment as described inExample 2, if desired, to further comminute the discrete particlesproduced.- 4 v M EXAMPLE 4 are produced in'a-direct operation bysubjecting'a bar made. of the alloy to repeated rolling under pressurewithout annealing the alloy between the passes of the roller. Theprocess gives the alloy a brittle surface due to strain hardening, andthe surface is then easily broken up into a powder consisting ofwafer-shaped particles. Oxidation follows, as described above.

Further modifications and refinements of the process of this inventionwill be obvious to those skilled in the art. The above description andexamples should not, therefore, be construed in a limiting sense.

I claim:

1. A method for making silver/cadmium oxide electrical contactmaterials, which comprises converting a silver/cadmium alloy stockmaterial incorporating from 80 to 95 weight percent silver and from to20 weight percent cadmium, to discrete thin particles whose majordimensions are at least three times their thickness, and which have asubstantially uniform thickness of no more than 5 microns, and heatingsaid particles in an oxygen rich atmosphere to oxidize the cadmiumcontent thereof and produce a substantially homogeneous silver/ cadmiumoxide contact-forming composition.

2. The method of claim 1, in which the discrete particles of thesilver/cadmium alloy are formed by melting said alloy stock material,and impinging a stream of said molten material under pressure against asubstantially planar surface to flatten and comminute the particles tothe desired dimensions.

3. The method of claim 1, in which a solid body constituted of thesilver/cadmium alloy is reduced to said discrete silver/cadmiumparticles by abrading said bed: to remove flake-like particles havingthicknesses of fron 1 to 3 microns therefrom.

4. The method of claim 1, in which a solid body constituted of thesilver/cadmium alloy and having a substantially planar and smoothsurface is subjected to repeated rolling under pressure, withoutintermediate annealing, to form the desired flake-like alloy particles.

5. The method of claim 1, wherein said particles are heated attemperatures of from 1300 to 1550 F. for periods of from 2 to 36 hoursto eifect the substantially complete oxidation of the silver/cadmiumalloy to a homogeneous silver/cadmium oxide material.

References Cited UNITED STATES PATENTS 2,796,346 6/1957 Stumbock -1732,861,155 11/1958 Farnham 75173 X 3,114,631 12/1963 Sistare 29-182.5 X3,317,991 5/1967 Haarbye 75--206 X BENJAMIN R. PADGE'IT, PrimaryExaminer A. J. STEINER, Assistant Examiner US. Cl. X.R.

